A shifting wet clutch incorporated into a vehicle typically uses an automatic transmission fluid to lubricate and facilitate operation of the shifting wet clutch or other transmission components. Through use of the transmission and operation of the shifting wet clutch, the automatic transmission fluid progressively degrades. A maintenance strategy for vehicles equipped with the shifting wet clutch includes replacement of the automatic transmission fluid upon degradation. However, as the degradation process of the automatic transmission fluid is very complex, determining when the automatic transmission fluid should be replaced is difficult, and complicates the maintenance strategy of the vehicle. A monitoring and prognostics system for determining when the automatic transmission fluid should be replaced without significantly increasing a cost of the vehicle the monitoring and prognostics system is incorporated in would be advantageous.
The degradation process of the automatic transmission fluid comprises a plurality of separate processes. Non-limiting examples of the plurality of separate processes are oxidation, thermal breakdown, microdieseling, electrostatic spark discharge, additive depletion, water contamination, and excessive shear load. An example of the degradation process of an automatic transmission fluid is illustrated in FIG. 1.
Oxidation is a chemical process caused by exposure to oxygen. Besides an oxygen concentration in a surrounding environment, oxidation is also influenced by other factors, such as a temperature of the surrounding environment and a presence of a catalyst which may increase a rate of oxidation. Non-limiting examples of the catalyst are water and wear metal ions.
Oxidation is the most predominant process that contributes to the degradation process. Oxidation of the automatic transmission fluid results in an increase in a viscosity of the automatic transmission fluid, formation of a varnish, sludge and sediment within the automatic transmission fluid, a depletion of additives in the automatic transmission fluid, a breakdown of a base oil of the automatic transmission fluid, a plugging of a filter used with the automatic transmission fluid, a loss in foaming properties of the automatic transmission fluid, an increase in an acid number of the automatic transmission fluid, an ability of rust to form on the components of the transmission, and a corrosion of the components of the transmission.
Thermal breakdown can cause the automatic transmission fluid to vaporize or the automatic transmission fluid to decompose. Vaporization or decomposition can cause additives to be removed from the automatic transmission fluid, resulting in a loss of function of the automatic transmission fluid. Further, as a result of the additives being removed from the automatic transmission fluid, the viscosity of the automatic transmission fluid may increase. One source of thermal breakdown is from friction which occurs between the moving parts of the transmission. Friction may cause the automatic transmission fluid to heat up above a recommended stable temperature. As a non-limiting example, the recommended stable temperature may be a flash point of the automatic transmission fluid.
Thermal breakdown occurs when a temperature of the automatic transmission fluid greatly exceeds the recommended stable temperature of the automatic transmission fluid, larger molecules of the automatic transmission fluid will break apart into smaller molecules. Thermal breakdown can trigger side reactions, such as inducing polymerization, producing gaseous by-products, destroying additives and generating insoluble by-products. In some cases, the viscosity of the automatic transmission fluid may decrease due to thermal breakdown.
Microdieseling, which may also be referred to as pressure-induced thermal degradation, is a process in which an air bubble transitions from a low-pressure to a high-pressure zone, resulting in adiabatic compression. Such a process produces localized temperatures in excess of 1,000° C., which results in a formation of carbonaceous byproducts and accelerated degradation of the automatic transmission fluid.
Electrostatic spark discharge can occur when clean and dry automatic transmission fluid rapidly flows through tight clearances in the transmission. Internal friction in the automatic transmission fluid generates static electricity which can accumulate until a spark occurs. Such a spark occurs at an estimated temperature of between 10,000° C. and 20,000° C.
Due to a natural consequence of aging, some additives of the automatic transmission fluid are depleted. Monitoring additive levels is important not only to assess the health of the automatic transmission fluid, but it also may provide information related to specific degradation mechanisms.
Water contamination in the automatic transmission fluid occurs when water is dissolved in the automatic transmission fluid. When this occurs, water molecules are dispersed evenly within the automatic transmission fluid. When a maximum level of dissolved water in the automatic transmission fluid is reached, microscopic water droplets are uniformly distributed in the automatic transmission fluid to form an emulsion. When additional water is added to the emulsion, the two components will become separated, which results in free water in the automatic transmission fluid
Effects of water in the automatic transmission fluid include rust and corrosion to the components of the transmission, erosion to the components of the transmission, water etching to the components of the transmission, and hydrogen embrittlement of the components of the components of the transmission. In addition water can also accelerate the oxidation process, deplete oxidation inhibitors and demulsifiers added to the automatic transmission fluid, precipitate additives added to the automatic transmission fluid and compete with polar additives such as friction modifiers for components of the transmission.
Excessive shear load is another process that contributes to the degradation process of the automatic transmission fluid. Excessive shear load can scissor a molecular chain of viscosity modifiers of the automatic transmission fluid, resulting in a permanent viscosity loss of the automatic transmission fluid.
Some indicators revealing quality of the automatic transmission fluid after use for long periods of time have been reported. These indicators can be classified as follows:
When the automatic transmission fluid is clear and bright, this indicates that there are no deposits, and the automatic transmission fluid is still in good condition. When automatic transmission fluid becomes darker in color, such color generally indicates that the automatic transmission fluid has been in use for a long period of time and/or that some oxidation of the automatic transmission fluid has occurred.
The viscosity is a measure of a resistance to flow, or how thick or thin the automatic transmission fluid is. The viscosity of automatic transmission fluid is an important property of the automatic transmission fluid which is indicative of performance. A decrease in the viscosity of the automatic transmission fluid can be caused by an admixture with a lower viscosity oil, a breakdown of polymeric compounds of the fluid viscosity index from excessive shear loads or thermal breakdown. An increase of the viscosity of the automatic transmission fluid can be caused by an admixture with a higher viscosity oil, contamination of the automatic transmission fluid with heavy fuel, combustion products, or products from incomplete combustion (such as soot, solids, coolant and water) or oxidation.
The ability of the automatic transmission fluid to combat effects of acid formation is indicated by a total base number (TBN) of the automatic transmission fluid. Acid is typically formed during the oxidation process. A low TBN can indicate an admixture with a lower TBN oil, or depletion of additives. Such depletion of additives may indicate that the automatic transmission fluid is exposed to severe conditions such as high sulphur fuel, over-extended oil drain, inappropriate transmission design, and harsh operating conditions. A high TBN can indicate an admixture with a higher TBN oil, a decomposition of alkaline additives of the automatic transmission fluid by high temperature, and a stripping of light ends from the automatic transmission fluid.
Acidity of the automatic transmission fluid is an indicator of serviceability. Acidity of the automatic transmission fluid increases with oxidation and the introduction of combustion byproducts. Acidity of the automatic transmission fluid may be measured using a total acid number (TAN) of the automatic transmission fluid. A lower TAN may indicate an admixture of the automatic transmission fluid with a lower TAN oil, depletion of additives contributing to the TAN of the automatic transmission fluid, and contamination of the automatic transmission fluid with alkaline oils or other materials (such as cleaning agents, for example). A higher TAN may be caused by an admixture of the automatic transmission fluid with a higher TAN oil, oxidation of the automatic transmission fluid, contamination of the automatic transmission fluid with acid combustion products, a thermal breakdown of certain additives, and contamination of the automatic transmission fluid with acidic materials (such as cleaning agents, for example). The higher TAN of the automatic transmission fluid can produce corrosion of the components of the transmission, thickening of the automatic transmission fluid, formation of deposits within the transmission, and an accelerated wear of the components of the transmission. It is understood that the TBN and the TAN are complementary. Further, the TAN and a viscosity of an automatic transmission fluid may vary over a course of time, as can be seen in FIGS. 3A-3C, which have been based on information from “A comprehensive look at the acid number test” by the Noria Corporation. Practicing Oil Analysis, 2007: 1-9.
Causes of water presence in the automatic transmission fluid include condensation, low operating temperatures of an engine, leakage of other components into the transmission, ingress into the transmission, water present in new automatic transmission fluid, and a mixing of the automatic transmission fluid with combustion gases. The effects of water in the automatic transmission fluid are rust and corrosion of the components of the transmission, a sludge formation within the transmission, an increase in oxidation of the automatic transmission fluid, a decrease in fatigue life (typically of bearings and gears of the transmission), an increased wear of the components of the transmission, a reduced additive protection, and plugging of the filter of the automatic transmission fluid.
Solids in the automatic transmission fluid can come from many sources, but common causes include wear debris particles from the friction plates, gears, and bearings of the transmission, products of the oxidation process, and ingress of environmental products into the transmission (such as sand and soil, for example). The effects of high solids content are a thickening of the automatic transmission fluid, abrasive wear of the components of the transmission, formation of deposits with the transmission, and plugging of the filter of the automatic transmission fluid.
As mentioned hereinabove, the degradation process which occurs in the automatic transmission fluid is very complex. Many factors are involved in determining the degradation of the automatic transmission fluid, such as an operating condition and an environment of the transmission. However, it is known that the oxidation process is the major mechanism causing degradation of the automatic transmission fluid. As a result of the oxidation process, the automatic transmission fluid becomes more acidic and more viscous. As a result, the total acid number and the viscosity are two metrics that may be used to determine a condition of the automatic transmission fluid.
It would be advantageous to develop a system and a method for determining a remaining useful life of lubricant in a wet clutch transmission that is accurate and is based on data from sensors which are available in the transmission without significantly increasing a cost of the transmission or a vehicle the transmission is incorporated in.